The invention relates to a membrane for the separation of liquid mixtures according to the pervaporation principle whereby the membrane consists of a swellable elastomeric polymer or polymer blend (polymer material) in which is incorporated a membrane-stabilizing, respectively reinforcing support that is essentially parallel to the membrane surface, the membrane having a feed side and a permeate side.
A membrane of the aforementioned kind has been known from U.S. Pat. No. 3,770,567. This known membrane is a so-called ion exchange membrane, i.e., with this known membrane the separation of the liquid mixture is effected according to the electrodialysis principle. This known ion exchange membrane is provided with a support material so that it may withstand greater pressures of the liquid mixture to be separated than unsupported membranes. A complete inclusion of fabric fibers of the support into the polymer material is thereby an essential condition for a structural reinforcement of the membrane without any imperfections whereby the polymer material may be thinner locally than corresponds to the fabric layer. The linear expansion due to swelling of the known ion exchange membrane is reduced from 14 to 17% (without support) to 3% when the fabric-type support is employed.
However, in membranes with which a separation of liquid mixtures according to the principle of pervaporation is carried out, the liquid mixture to be separated is fed to the membrane in a cross flow mode, whereas the permeate exiting from the membrane is removed in the form of vapors either by applying a vacuum to the backside (permeate side) of the membrane or with the aid of a circulating carrier gas stream. The separation of the mixture components is achieved due to their different membrane permeability and may be performed even against the ratio of vapor pressures, i.e., favoring the less volatile component. The membrane permeability is composed of the sorption of the liquid feed mixture into the membrane, the diffusion through the membrane, and the desorption at the backside of the membrane into the permeate vapor compartment. The sorption, in this context, has the greatest impact on the separation efficiency, i.e., the separation depends primarily on the preferred sorption of one component of the mixture relative to the other or others by the polymer membrane. The liquid sorption by the polymer membrane is associated with swelling of the membrane. The membrane swelling, which increases with the concentration of the preferentially permeating component in the feed mixture, is described by the so-called sorption isotherms, which, in the simplest case, predict a linear increase of the degree of swelling depending on the feed concentration.
A preferred application of the membrane separation by pervaporation is the separation of aqueous-organic liquid mixtures. In particular, aqueous-organic liquid mixtures comprised of water containing a relatively minor amount of organic components, are widely encountered as organically laden waste waters and as process effluents in biotechnology. The separation of such mixtures with the selective removal of the minor amounts of organics is performed by pervaporation via membranes of elastomeric polymers (polymer material) such as, for example, silicone rubber (polydimethylsiloxane), polyurethane, or polyether-polyamide-copolymers. In contrast to evaporation, pervaporation allows the selective removal of organic compounds having a considerably higher boiling point than water from water.
A prerequisite for a favorable separation effect, expressed as the enrichment of the organic component in the permeate in connection with its depletion (removal) in the retentate, is the selective sorption capacity of the membrane polymer for the organic component of the mixture, which is manifested by swelling. Elastomeric polymers have an especially strong swelling tendency which, in itself, is a condition for the desired separation effect. In summarizing the above, it may be said that with elastic membranes of a polymer, respectively, a polymer blend, employed according to the principle of pervaporation, a hindrance of the swelling would also result in a reduction of the sorption capacity of the membrane for the components to be pervaporated, resulting, in return, in a reduction of the separation effect of the membrane.
In the aforementioned ion exchange membrane according to the principle of electrodialysis (U.S. Pat. No. 3,770,567), the membranes are embodied relatively thick which is basically disadvantageous for a mixture separation according to the principle of electrodialysis. For this reason, the known ion exchange membrane was provided with a fabric reinforcement for the aforementioned reasons of improved handling properties and the desired greater pressure stability in order to counteract the membrane swelling. In the known ion exchange membrane the aforementioned measures reduced the swelling from between 14 to 17% to 3% which is exactly contrary to the objective to be realized by this invention.
It is an object of the present invention to provide a membrane for the separation of liquid mixtures which yields freely to the swelling pressure due to pervaporation without resulting in substantial changes to the surface area dimensions of the membrane, whereby the membrane is able to withstand high concentrations of swelling-inducing components in the water as well as operational changes of the swelling stage, whereby no foreign resistance counteracts the removal of the permeate vapors from the membrane, and whereby the flux through the membrane, relative to conventional membranes, is increased and the selectivity of the membrane is improved. Furthermore, the membrane shell may be manufactured according to conventional manufacturing methods for known membranes so that they may be produced in a simple and inexpensive way.